1. Field of the Invention
This invention relates to a process for producing a sheet or strip of an austenitic stainless steel, particularly one containing 18% Cr and 8% Ni. More particularly, it is concerned with a process which does not include annealing of a hot rolled strip, and yet can produce a product of excellent quality having a satisfactorily low degree of anisotropy.
2. Description of the Prior Art
A conventionally known process for producing a strip of an austenitic stainless steel, particularly one containing 18% Cr and 8% Ni, comprises hot rolling a slab, softening a hot rolled strip at a temperature of at least 1,000.degree. C., descaling it, cold rolling it once or twice, annealing the cold rolled strip, and pickling it. If the strip is to be cold rolled twice, it is annealed after the first rolling. The hot strip annealing, and descaling are performed by a line known as a HAP line, designed specially for handling stainless steel.
The hot strip annealing step requires a high temperature, and a large amount of energy for softening the steel and forming a solid solution of a carbide, so this step has a significant bearing on the operating speed of the HAP line. Accordingly, eliminating this step contributes greatly to saving energy and improving productivity.
A study made by the present inventors that the hot strip annealing step is useful for the following purposes:
(1) Recrystallizing and softening the steel sufficiently to render it suitable for cold rolling; PA1 (2) Forming a solid solution of a carbide precipitated during the steps of hot rolling and coiling; PA1 (3) Raising the speed of pickling; and PA1 (4) Improving the anistropy of the product.
Point (1) is discussed in Japanese Laid-Open Patent Specification No. 70404/1980, and point (2) in Japanese Laid-Open Patent Specifications Nos. 77523/1976 and 107729/1980. There are, however, still a lot of problems to be solved. The inventors known of hardly any prior study on the latter two points (3) and (4).
The inventors noted that a stainless steel strip which had not been annealed prior to cold rolling exhibits a higher degree of anisotropy than one which had been annealed. A high degree of anisotropy brings about a great difference in properties from one portion of the strip to another, i.e., in the rolling direction, in a direction perpendicular to the rolling direction, and in a direction having an angle of 45.degree. to the rolling direction. This difference may, for example, result in a high ratio of "earing" or ear formation on a deep drawn cylindrical product, and therefore, a low yield of produciton. The earing ratio can be expressed by the following equation: ##EQU1## in which h.sub.max and h.sub.min represent the dimensions shown in FIG. 1. A cylindrical deep drawing test for the evaluation of a strip for earing tendency may be conducted by employing a strip thickness of 0.7 mm, a blank diameter of 80 mm and a punch diameter of 40 mm. It has been found experimentally that a strip which has not been annealed prior to cold rolling exhibits an earing ratio of, say, 10%, while a strip which has been annealed shows an earing ratio of, say, 4 to 6%.